As to the Vienna data on mercury and gold, Dr. Ehrenhaft publishes, all told, data on just sixteen particles and takes for his Brownian-movement calculations on the average fifteen times of fall and fifteen of rise on each, the smallest number being 6 and the largest 27. He then computes his statistical average from observations of this sort. Next he assumes Perrin’s value of , namely, , which corresponds to , and obtains instead by the Brownian-movement method, i.e., the method, the following values of , the exponential term being omitted for the sake of brevity: 1.43, 2.13, 1.38, 3.04, 3.5, 6.92, 4.42, 3.28, .84. Barring the first three and the last of these, the mean value of is just about what it should be, namely, 4.22 instead of 4.1. Further, the first three particles are the heaviest ones, the first one falling between his cross-hairs in 3.6 seconds, and its fluctuations in time of fall are from 3.2 to 3.85 seconds, that is, three-tenths of a second on either side of the mean value. Now, these fluctuations are only slightly greater than those which the average observer will make in timing the passage of a uniformly moving body across equally spaced cross-hairs. This means that in these observations two nearly equally potent causes were operating to produce fluctuations. The observed ’s were, of course, then, larger than those due to Brownian movements alone, and might easily, with but a few observations, be two or three times as large. Since [Pg 171] appears in the denominator of equation (29), it will be seen at once that because of the observer’s timing errors a series of observed ’s will always tend to be larger than the due to Brownian movements alone, and hence that the Brownian-movement method always tends to yield too low a value of , and accordingly too low a value of . It is only when the observer’s mean error is wholly negligible in comparison with the Brownian-movement fluctuations that this method will not yield too low a value of . The overlooking of this fact is, in my judgment, one of the causes of the low values of recorded by Dr. Ehrenhaft.